1. Field of the Invention
This invention relates to a process for producing trialkoxysilanes. In particular, it relates to a process for producing trialkoxysilanes in which elemental silicon is reacted with an alcohol in the presence of a copper catalyst to obtain the desired products in a high yield.
2. Description of the Prior Art
Trialkoxysilanes (e.g., trimethylsilane, triethylsilane, etc.) composed of a silicon atom bonded with three alkoxy groups and one hydrogen atom are highly unstable and reactive and, hence, undergo various reactions, such as addition, copolymerization, copolycondensation, disproportionation, etc., with other organic compounds, thereby yielding a variety of useful substances which can be utilized as raw materials for making silane coupling agents, coating agents, heat-resistant paints or silane gas to be used in the production of semiconductors.
Known processes for producing trialkoxysilanes include the one in which trichlorosilane is reacted with an alcohol in accordance with the following equation: EQU HSiCl.sub.3 +3ROH.fwdarw.HSi(OR).sub.3 +3HCl
(in which R represents an alkyl group) and the one in which elemental silicon is allowed to react with an alcohol in gas or liquid phase in the presence of a copper catalyst, whereby complicated reactions, including the following:
(1) Si+4ROH.fwdarw.Si(OR).sub.4 +2H.sub.2 PA1 (2) Si+3ROH.fwdarw.HSi(OR).sub.3 +H.sub.2 PA1 (3) Si+2ROH.fwdarw.H.sub.2 Si(OR).sub.2 PA1 (4) Si+2ROH.fwdarw.SiO.sub.2 +2RH PA1 (5) ROH+H.sub.2 .fwdarw.RH+H.sub.2 O PA1 (6) Other products: ##STR1## (in which R represents an alkyl group) take place, depending on reaction conditions (see, e.g., Inorg. Chem., Vol. 9, No. 5, 1071-1075 (1970)). PA1 (1) Si+2RCl.fwdarw.RSiCl.sub.3 +R.sub.2 SiCl.sub.2 +R.sub.3 SiCl PA1 (in which R is an alkyl group, such as methyl, ethyl, etc.) PA1 (2) (RSiCl.sub.3 +R.sub.2 SiCl.sub.2 +R.sub.3 SiCl)+ROH.fwdarw.R.sub.x Si(OR).sub.y Cl.sub.z +yHCl PA1 (in which x, y and z are 1, 2 or 3) PA1 (3) RCl+MOR.fwdarw.MCl+ROH PA1 (in which M is a metal, such as Ca, Al, Zn, Mg, Na, K, Ni, Ba, etc.) PA1 (4) 2Cu+RCl--CuCl+RCu
In the former process, desired trialkoxysilanes can be obtained only in a poor yield. The process also suffers from the problem that reactors used for the production are corroded by hydrogen halogenides produced as a by-product. It is thought that the corrosion occurs in accordance with the following mechanism: The by-product, i.e., hydrogen halogenides, react with alcohols to produce alkyl halides and water. The water so-produced hydrolyzes the raw material (trichlorosilane) and the products (tri- or tetra-alkoxysilanes), thereby forming polycondensation products.
In the latter process, alcohols are used in general in an excessive amounts in order to increase the reaction percentage (i.e., percentage reacted) and the reaction velocity of elemental silicon. In the case of a gas phase reaction, the reaction shown by the above formula (1) takes place predominantly, and reactions shown by the above formulas (4) (5) also take place because of high reaction temperature. Water formed in accordance with reaction formula (5) reacts with elemental silicon to form a film of silicic acid or silicon oxide (silica), or with the copper catalyst to form a copper oxide. Because of this, both of them become inactive and the reactions terminate midway, thus causing the problems that the reaction percentage of elemental silicon becomes lower and that the desired trialkoxysilanes can be obtained in a poor yield.
In cases where the latter process is carried out in a liquid phase, the reaction shown by formula (2) proceeds preferentially at the initial stage of the reaction and, hence, the desired trialkoxysilanes can be obtained with a good selectivity. However, with the progress of the reaction, the selectivity of trialkoxysilanes decreases gradually. It is therefore impossible to obtain the desired trialkoxysilanes in a satisfactorily high yield. The reason why the selectivity of trialkoxysilanes decreases with the progress of the reaction is not certain. It may however be explained as follows. Metallic elemental silicon used in industries usually contains various impurities, for example, alkali metals and/or alkaline earth metals, in an amount not greater than ca. 1% by weight. Such impurities react with alcohols to form metal alcolates which gradually accumulate with the progress of the reaction. With the increase in the quantity of accumulated metal alcolates, the pH value of the reaction system gradually shifts from acidic to neutral or basic. Because of this, once formed trialcoxysilanes are converted into tetraalkoxysilanes. With regard to the yield, it is considered to be important to activate elemental silicon and copper catalysts to the highest level possible and to suppress the above-described reactions (4) and (5) to the lowest level possible, by carrying out the reaction at a relatively low temperature.
The problems in the above liquid phase reaction can be solved by maintaining the acidity in the reaction system at a constant level, by using highly pure elemental silicon which is substantially free from the undesirable side reactions of alcohols with the impurities. However, purification of elemental silicon is highly time consuming and costly and, hence, it is virtually impossible to use highly pure elemental silicon which is pure enough not to cause the adverse effects in the production of trialkoxysilanes on a commercial scale.
The process in which elemental silicon is directly reacted with alcohols in gas or liquid phase in the presence of a copper catalyst also suffers from the problem that the reaction percentage of the alcohols is not high enough and, hence, the resulting reaction mixture contains large quantitis of unreacted alcohols, together with the desired trialkoxysilanes and other alkoxysilanes, such as tetraalkoxysilanes, dialkoxysilanes, dialkoxyalkylsilanes, trialkoxyalkylsilanes and dimers of these. In addition, the unreacted alcohols contained in the reaction mixture react with the desired trialkoxysilanes, thereby forming tetraalkoxysilanes, and the desired trialkoxysilanes undergo disproportionation to form dialkoxysilanes and tetralkoxysilanes in the course of purification or during storage before purification. As a result, the yield of trialkoxysilanes is markedly lowered.
Various methods have been proposed to solve the above problems. For example, the reaction product is stored at a temperature lower than 0.degree. C., or a certain amino acid is added to the reaction mixture for its stabilization (see Japanese patent application (Laid Open) No. 72,197/80); an amine is added to the reaction mixture and its pH is adjusted to 2.0 to 7.0 (see Japanese Patent Application (Laid Open) No. 118,592/82; or trivalent organic phosphates (Japanese Patent Application (Publication) No. 21,478/86) or epoxides (Japanese Patent Application (Laid Open) No. 1,694/86) are added to the reaction mixture.
However, in cases where the temperature of the reaction mixture obtained is stored at 0.degree. C. or below, refrigerators or freezers are required for its cooling. This makes its storage quite costly. In addition, the reaction mixture must be heated at the time when it is to be refined by means of rectification, which most likely results in decomposition of trialkoxysilanes. In cases where amino acids, amines, trivalent organic phosphates or epoxides are used as a stabilizer, refined trialkoxysilanes obtained by the distillation of the reaction mixture tend to be contaminated by the stabilizers used. In addition to this, those stabilizers did not always perform satisfactorily in preventing the reaction of trialkoxysilanes with unreacted alcohols. Or the disproportionation of trialkoxysilanes at the time of rectification of the reaction product, could not be sufficiently suppressed by the use of those stabilizers. In any case, the known stabilizers are by no means satisfactory.
In view of the above, the present inventors have conducted intensive investigations. As a result, it has now been found that, in the production of trialkoxysilanes comprising an activation step where elemental silicon and a copper catalyst are activated, a reaction step where an alcohol is contacted with elemental silicon and the copper catalyst so as to allow it to react with the elemental silicon and a purification step where the reaction mixture obtained is refined, the lowering in the selectivity of trialkoxysilanes can be prevented during the progress of the reaction, elemental silicon can be reacted at a high percentage and the trialkoxysilanes contained in the reaction mixture can be stabilized, by introducing a halogenide into, and allowing it to contact with, the reaction system and/or mixture in one or more of said steps.